The present invention relates generally to variable valve control systems for internal combustion engines which are capable of continuously varying a lift characteristic (a lift and a working angle) of an intake valve or an exhaust valve, and more particularly to a variable valve control system which is capable of calibrating a sensor for measuring the lift characteristic.
In recent years, there have been proposed and developed various variable valve control systems enabling both a working angle (an operating angle) and a phase to be varied for a high degree of freedom of valve lift characteristics and enhanced engine performance through all engine operating conditions. The variable valve control systems are applied to internal combustion engines, especially to gasoline engines which generate output power in accordance with the quantity of intake air, in order to enhance engine performance such as fuel economy. Some variable valve control systems include a variable lift and working angle control mechanism to continuously expand or contract a valve lift and a working angle of an intake valve, and a variable phase control mechanism to retard or advance the angular phase at the maximum intake-valve lift point (often called “central-angle phase”). Such a variable valve control system is capable of controlling the quantity of intake air flowing into a combustion chamber of each of the cylinders without the control of the opening of a throttle valve. Such variable valve control systems have been disclosed in Japanese Patent Provisional Publications Nos. 2002-349215 (hereinafter is referred to as “JP2002-349215”) and 2003-41955 (hereinafter is referred to as “JP2003-41955”). In the systems disclosed in JP2002-349215 and JP2003-41955, the variable actuation mechanism continuously changes the valve lift characteristic in accordance with the angular position of a control shaft. In order to precisely control the valve lift characteristic, the variable valve control system includes a control shaft sensor to detect the angular position of the control shaft. The variable valve control system calibrates the control shaft sensor by correcting information concerning a reference position for measuring and outputting the angular position of the control shaft. The calibration is executed in a condition where the control shaft is in a mechanical limit position defined by a stopper, and a torque necessary to drive the valve train is small. That is, the calibration is executed while the lift characteristic is a mechanically defined minimum lift.